Industrial process control systems have often required the measurement of the electrical conductivity of materials, such as fluids, used in the system. In the past, these measurements have been made by using sensors which fall into one of two categories, i.e., contacting sensors and non-contacting sensors. Contacting sensors rely on the electrical contact of the measurement electronics to the material, e.g., a conductive fluid, via one or more sets of electrodes positioned in the material at fixed distances from each other. A voltage is applied across one set of electrodes and the induced current in the material is measured either across the electrodes or alternatively through a second set of electrodes, the measured flow of electrical current being proportional to the conductivity. Non-contacting sensors utilize two inductors, or transformers, one of which is used to induce voltage in the material and the second one of which is used to measure the resultant flow of current which occurs in the material.
In the past, implementations of such systems often are not satisfactory, particularly when using non-contacting sensors, because of the inherent lower sensitivity of inductive sensors, especially when measuring very low conductivities. Moreover, prior art non-conducting sensors normally have large physical geometries which are needed to overcome the sensitivity limitations of smaller sensor configurations.
In addition, in prior systems, measurement errors arise which are due to errors generated by the electronic circuitry that is used in such systems. Prior systems normally do not effectively address such problems when using either type of sensors and, hence, the accuracy of the measurements made by such systems is unacceptable in many applications. In the prior art, the electronics are typically configured as shown in FIG. 1 as a "feed forward" circuit where a signal generator 10 is used to drive a voltage Vi across a drive coil 11, which results in a current Is flowing in a material, e.g., a conductive solution 12, represented as flowing in a loop having a solution conductivity I/Rs, which current is proportional to the conductivity Gs of the solution. The presence of the current Is results in a current Im flowing in the sensor coil 13 which is measured by a suitable current meter 14 and found to be proportional to the conductivity of the solution. In this circuit there are many sources of errors, many of which are related to the magnetic properties of the transformers, which errors have limited their accuracy in the past.
It is desirable to devise a system which can utilize sensors in a manner so as to provide a highly accurate measurement of electrical conductivity even at relatively low conductivity levels, and even when using relatively small non-contacting sensors.